Hi there and welcome to Just answer!! I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
These swellings are probably caused by swollen lymph nodes. These can swell due to allergies or infections that drain from these areas. If there are any teeth issues, these will drain through the lymph nodes causing them to be swollen. It is very rare for salivary glands to be come swollen throughout.
Since these occur sporadically and Nachos is still eating well and active, this is probably caused by allergies....this will respond to anti histamines that your vet can prescribe for you.
Good Luck! Sincerely, XXXXX XXXXX